I have been trying to install OpenBSD on my netbook for 3 days now without any success.

I have tried dd-ing the floppy image to usb drive, it didnt work.
I have tried creating a LiveUSB under VirtualBox running OpenBSD.
Then it seemed I found an easy solution using UNetbootin to create a bootable USB from install47.iso. However, when its bootloader starts it immedieatley says Invalid or corrupt kernel image.
I checked the sha256sums and they are matching...

So now I'm really lost about what to do to install OpenBSd on my netbook.

I don't prefer solutions like the following, but for those who don't already have an OpenBSD installation which they can leverage to build USB install media, there aren't a lot of choices:

http://liveusb-openbsd.sourceforge.net/

jggimi's live CD may also be of use:

http://jggimi.homeip.net/

First of all let me thank you for your fast help! Much appreciated!

However, http://liveusb-openbsd.sourceforge.net/ this solution does not seem to work. I downloaded the install image, dd-d it and I happened to have qemu and tested it and it seemed fine. However when I rebooted it just straight away ignored the USB and jumped to GRUB. I even tried disabling booting from HDD but it insists to insert a valid boot media.

Now, I'm trying the another solution, hope will have better luck with that one.

I hope I won't run into any other trouble during the installation tomorrow.

I, too, have installed OpenBSD on various netbooks, & figuring out the incantation needed to boot from USB was usually the strangest of the idiosyncrasies encountered.

If you maintain a notebook of hardware/software oddities, the solution you found today may be worthy of noting. Otherwise, you may be going through the same frustration & discovery again if you ever have to install in the future.

As for OpenBSD's installation process, be sure you understand what is being asked for where. Studying Section 4 of the official FAQ before embarking on installation will save you much frustration & bewilderment.

In fact, you will save yourself significant frustration by taking the time now, as a newcomer, to studying all of the FAQ. It is the single best document which describes the current state of OpenBSD.

Many of the questions we encounter on these forums are from those that don't read the FAQ. Yes, some beginner documents & FAQ's are poorly written, but this one is solid. The OpenBSD project is small, so investing the time to write one document well solves a number of problems all at once.